Our objective is to comprehend the functioning of certain fibrous protein assemblies whose important feature is that they have both structural and dynamic roles. Our chief methods will be coordinated X-ray diffraction and electron microscopy. Muscle is a major focus and provides the background for studying other related systems. Two key questions are: How is contractile force generated? And how is it switched on and off? Crystals of tropomyosin and the regulatory complex tropomyosin/troponin are being analyzed to relate the detailed structure of these molecules to their role in muscle. Comparative studies of intact muscle and in vitro aggregates of the muscle proteins are being undertaken to obtain a mechanism for the myosin switch. We are also analyzing X-ray fiber diagrams of microtubules to obtain a detailed picture oF the microtubule and its assembly. We are determining the structure of fibrinogen by a crystallographic analysis in order to relate the morphology of the molecule to its chemical sequence and to establish its packing in the fibrin clot. Our broad study of these assemblies is directed towards identifying the distinctive features of their architecture that are essential to their function.